This invention relates to a process for producing isobutylene by dehydration reaction of t-butyl alcohol.
Isobutylene is used as a starting material for polymers such as butyl rubber, polybutene, etc. It is also used as starting material for industrial products such as isoprene, BHT, t-butylcatechol, t-butyl esters of various carboxylic acids, etc. Recently, it is noticed as source of starting materials for production of methyl methacrylate.
It has been well known that isobutylene is produced by dehydration of t-butyl alcohol in the presence of an acid catalyst.
The dehydration reaction in liquid phase is usually carried out in a homogeneous system using a strong acid such as sulfuric acid. However, this method requires expensive anticorrosion equipment and furthermore produces waste acids and is industrially not preferred due to these problems.
Recently, for solving these problems, it has been proposed to effect the above dehydration reaction in heterogeneous systems using strongly acidic ion exchange resins containing sulfonic acid group as a catalyst (U.S. Pat. No. 4,012,456, EP 82,937 and Japanese Patent Kokai Nos. 135710/79 and 138506/79). However, proportion of water increases under high conversions, resulting in reduction of reaction velocity and polymerization of isobutylene, a side-reaction, proceeds to cause decrease of yields, since t-butyl alcohol is in equilibrium with isobutylene and water in reaction of liquid phase system. Thus, in order to obtain high yields, it is necessary to control the conversion rate and unaltered materials are recovered and recycled to the reaction system. This complicates the production process.
It is also well known that isobutylene is easily produced by bringing t-butyl alcohol into contact in gas phase with solid acids such as solid phosphoric acid, active alumina, silica-alumina, etc. at high temperatures. U.S. Pat. No. 4,036,905 discloses industrial production of isobutylene using this reaction where the reaction is effected at a high temperature of 246.degree.-413.degree. C. on a solid acid catalyst. In order to effect the dehydration reaction with high yields, it is necessary to restrain the side-reaction of polymerization of isobutylene and thus it is preferred to effect the reaction at higher temperatures. However, it is difficult to economically and inexpensively obtain heat sources for high temperatures and besides extra energy is required to heat the reaction gas to high temperatures and cool to temperatures for easy handling after completion of the reaction. Under the circumstances, there has been demanded a method according to which high yields is attained by carrying out the reaction economically and at lower temperatures.
However, there is no method to produce isobutylene from t-butyl alcohol or its aqueous solution in high yields at lower temperatures with highly active catalysts.
According to the process disclosed in Japanese Patent Kokai No. 13250/72, the dehydration reaction is effected at relatively lower temperatures of 50.degree.-300.degree. C. to attain high yields, but this patent publication is silent on selectivity of the dehyeration reaction. After the reaction was repeated with various catalysts using 90% aqueous t-butyl alcohol solution, it is found that selectivity of isobutylene is lower as activity of catalysts employed is higher and reaction temperature is lower and that the reaction is desirably carried out at higher reaction temperatures, preferably higher than 300.degree. C. in order to obtain high yields.
The inventors have made researches on various catalysts for economical production of isobutylene from t-butyl alcohol or inexpensively available aqueous solutions thereof as a starting material by gaseous phase dehydration reaction. As a result, it has been found that silica-alumina has high activity at low temperatures. However, with reference to the yield, it has been found that conversion rate and selectivity of reaction greatly vary depending on space velocity of reaction gas and reaction temperature and higher yields are difficult to obtain with decrease in reaction temperature.
Main factor for the reduction of selectivity under high conversion rate of reaction is polymerization reaction of isobutylene produced. The polymerization reaction of isobutylene can be restrained to some extent when reaction is carried out at higher temperatures, but it gradually proceeds at lower reaction temperatures to cause reduction of selectivity of isobutylene. If a process where the polymerization reaction of isobutylene can be restrained in spite of low temperature reaction. isobutylene can naturally be produced in high yields and furthermore, after-treatment of the produced gas can be easily accomplished by known method. Thus, this process will be very attractive as a process for industrial production of isobutylene.
The inventors have made researches separately on dehydration reaction of t-butyl alcohol in gaseous phase with use on silica-alumina catalyst and polymerization reaction of isobutylene.
The dehydration reaction of t-butyl alcohol proceeded very easily on silica-alumina catalyst and conversion rate enough for practical use was obtained at about 150.degree. C. However, with increase in conversion rate, selectivity of isobutylene abruptly decreased to cause conspicuous production of diisobutylene and furthermore triisobutylene. On the other hand, it was experimentally confirmed that polymerization reaction of isobutylene very easily takes place at about 150.degree. C. and at the higher temperatures it becomes difficult to occur.
As a result of the inventors' extensive researches on the conditions where polymerization of isobutylene is difficult to occur even at relatively low temperatures, it has been found that isobutylene can be obtained at high selectivity even under high conversion rate of reaction by incorporating a non-reactive gas and/or water vapor into feed gas or reaction gas or by diluting at least a part of silica-alumina catalyst with a carrier or by combination of these means.